classnames is a utility function to conditionally join CSS class names together.

Your task: Implement a classNames(...args) function that:

• Accepts strings, objects, arrays, or a mix of them.
• Returns a single space-separated string of class names.
• Ignores falsey values (false, null, undefined, 0, "").
• Flattens arrays recursively.
• Includes object keys if their values are truthy.

Examples

classNames('foo', 'bar'); 
// 'foo bar'

classNames('foo', { bar: true }); 
// 'foo bar'

classNames({ foo: true, bar: false, qux: true });
// 'foo qux'

classNames('a', ['b', { c: true, d: false }]); 
// 'a b c'

classNames(
  'foo',
  { bar: true, duck: false },
  'baz',
  { quux: true },
); 
// 'foo bar baz quux'

classNames(null, false, 'bar', undefined, { baz: null }, '');
// 'bar'